Cardiac rhythm management devices are implantable devices that provide electrical stimulation to selected chambers of the heart in order to treat disorders of cardiac rhythm. A pacemaker, for example, is a cardiac rhythm management device that paces the heart with timed pacing pulses. The most common condition for which pacemakers have been used is in the treatment of bradycardia, where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) that are permanent or intermittent and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate and/or artificially restoring AV conduction. Pacing therapy may also be used in treatment of cardiac conduction disorders in order to improve the coordination of cardiac contractions, termed cardiac resynchronization therapy. Other cardiac rhythm management devices are designed to detect atrial and/or ventricular tachyarrhythmias and deliver electrical stimulation in order to terminate the tachyarrhythmia in the form of a cardioversion/defibrillation shock or anti-tachycardia pacing. Certain combination devices may incorporate all of the above functionalities. Any device with a pacing functionality will be referred to herein simply as a pacemaker regardless of other functions it may be capable of performing.
Cardiac rhythm management devices such as described above monitor the electrical activity of heart via one or more sensing channels so that pacing pulses or defibrillation shocks can be delivered appropriately. Such sensing channels include implanted leads which have electrodes disposed internally near the heart, which electrodes may also be used for delivering pacing pulses or defibrillation shocks through pacing or shock channels. The signals generated from the sensing channels are intra-cardiac electrograms and reflect the time course of depolarization and repolarization as the heart beats, similar to a surface electrocardiogram (ECG). Implantable devices may also incorporate one or more subcutaneously disposed electrodes (e.g., on the surface of the device housing) into a sensing channel for generating an electrogram signal, referred to herein as a subcutaneous ECG. A subcutaneous ECG is more similar in its morphology characteristics to a surface ECG than is an intra-cardiac electrogram. The electrogram signals generated from the sensing channels of an implanted device, whether an intra-cardiac electrogram or a subcutaneous ECG, may be transmitted wirelessly to an external device where they can be displayed and analyzed in much the same manner as a surface electrocardiogram.